Hard disk drives (HDDs) rotate magnetic disks and drive head gimbal assemblies (HGAs) at high speed in response to requests for huge capacity, high recording density, and high-speed accessing. These cause fluctuation of air (turbulence) to buffet the magnetic disks and HGAs. This turbulence buffeting will disturb positioning a head for reading data which have been recorded with high density on a magnetic disk. This is because the turbulence occurs at random and it is difficult to estimate its magnitude and cycle so that swift and accurate positioning control will be complex and difficult. Also, the turbulence buffeting may cause a noise to impair the quietness of the device.
Another problem caused by influence of the air within the device due to the high speed rotation other than the foregoing, is increased electric power consumption. When a magnetic disk is rotated at high speed, the air around the disk is dragged and rotated together. On the other hand, the air apart from the magnetic disk remains still so that shearing force arises therebetween to become a load against the rotation of the disk. This is called a windage loss, which becomes larger as the disk rotates at higher speed. In order to rotate the disk at high speed against the windage loss, a motor will require a larger output and electric power.
Given that the above-described turbulence and windage loss are proportional to the density of the gas within the device, there is an idea to reduce the turbulence and windage loss by enclosing low-density gas instead of air in a hermetically sealed HDD. Hydrogen, helium, or the like is exemplified as the low density gas, but helium is optimum because it is effective, stable, and safe in considering actual use. HDDs with sealed helium gas can overcome the above-described problems and accomplish swift and accurate positioning control, electric power saving, and satisfactory quietness.
However, molecules of helium are extremely small and a diffusion coefficient of helium is large. Therefore, there has been a problem that enclosures used in common HDDs are poorly sealed so that helium gas leaks easily during normal use. In order to make it possible to hermetically seal low density gas like helium gas, a technique disclosed in U.S. Patent Publication No. 2005/0068666 (“Patent Document 1”) has been suggested as described below.
Patent Document 1 discloses a magnetic disk device in which a feedthrough for connecting an FPC assembly inside the enclosure and a circuit board outside the enclosure is attached to an opening of the base and the enclosure is hermetically sealed by a cover. With respect to the joint section of the base and the cover through which the helium inside the enclosure is likely to be leaked, the aluminum die-casted base and the aluminum cover are laser-welded so as to securely seal the joint section. With respect to the attaching section of the feedthrough through which the helium inside the enclosure is likely to be leaked, the feedthrough is constituted by a flange and a plurality of pins fixed to the flange with sealants of glass or the like, and the flange is soldered to the rim of the opening on the bottom surface of the base so as to completely seal the attaching section.
As described above, the pins of the feedthrough are fixed to the flange with sealants. Glass is frequently used as the sealants, but the thermal expansion coefficient of the glass is significantly different from the one of aluminum used in the base. In order to avoid a break in a sealant due to thermal stress, the thermal expansion coefficient of the flange is preferably between the ones of the sealants and the base.
Since the thermal expansion coefficient of the flange is different from the one of the base, large thermal stress is applied to a solder joint between the flange and the base due to change in environmental temperature. On the other hand, the solder used for joining the feedthrough and the base has small material yield stress. For this reason, it is important to assure sufficient solder joining reliability between the feedthrough and the base for the thermal stress accompanying changes in environmental temperature in use of an HDD.
Moreover, an HDD is requested to be operable under severe conditions such as at a temperature of 90° C. (363 K) or −40° C. (233 K) as well as at room temperature. Common lead-free materials are alloys mainly composed of tin (Sn). These alloys undergo phase transformation (transition) as described in “The Third Face of Tin”, Seizo Nagaskai, Kotai Butsuri, Solid Physics I (1967) pgs. 47-51, (“Non-Patent Document 1”). That is, if the operation temperature is lowered to −40° C. (233 K), a phase transformation from the β-Sn structure to the α-Sn structure occurs so that the solder joint gets likely to break for the helium gas to leak.
“The Simple Hexagonal to β-Sn Martensitic Transformation in Sn-(7.0-9.5) at % In Alloys”, Y. Koyama and H. Suzuki, Acta Metal., 37 (1989) pgs. 597-602 (“Non-Patent Document 2”) has reported that Sn-(7.0-9.5) at. % In alloys of the tin and indium (In) alloy system undergo martensitic transformation. If a martensitic transformation occurs between the simple hexagonal structure and the β-Sn structure, lenticular surface reliefs induced by the martensite are generated, which may develop a crack due to stress concentration caused by peaks and valleys of the reliefs. As a result, an HDD employing a structure with substituted helium has a possibility of helium gas leakage.
Japanese Patent No. 3562891 (“Patent Document 2”) entitled “Lead-free solder and method for using the same” has proposed lead-free solder made of tin alloy which is composed of 0.1 to 57% of bismuth or 0.1 to 50% of indium and the remain of tin and common impurities, characterized by that the tin alloy further includes 0.001 to 5% of cobalt, the percentage of the bismuth is 5 to 57% excluding 5 to 7%, and the percentage of the indium is 3 to 50% excluding 3 to 7% in claim 1 in the scope of claims. However, it does not disclose the optimum composition range of the tin alloy with respect to the composition range of indium in the tin alloy for the mechanical reliability in a broad temperature range.
Similarly, in Japanese Patent Publication No. 2007-105750 (“Patent Document 3”), a 1000-cycle test is conducted with cycle condition of −40° C. to 125° C., 30 minutes of retention time, and 5 minutes of cycle transformation time, as described in the paragraph
However, it does not propose the optimum composition range of a tin alloy in consideration of an aging effect caused by exposure at low temperature for a long time.
Consequently, a solder joint is demanded which is highly reliable in actual use of an HDD at −40° C. to 90° C., and does not break even after exposure at low temperature for a long time not to leak helium gas.